Most current bonding systems use chelating or mineral acids to remove the mineral from the smear layer and from the intact subsurface dentin in order to enhance penetration of the primer and the formation of a hybridized dentin layer (Nakabayashi, 1982). Demineralization by acids may be responsible for destabilization of the mineral-depleted collagen (Scott and Leaver, 1974; Okamoto et al., 1991). Bonding of composite restoratives with acetone-based, carboxylic acid-containing adhesives and a water-based primer to such acid-conditioned dentin dried after conditioning and rinsing results in significantly lower shear bond strengths (SB S) than bonding to dentin kept moist after conditioning (Kanca, 1992; Gwinnett, 1992; Racean et al., 1992; Swift and Triolo, 1992; Dickens, 1995).
Drying of dentin after conditioning with acids has been shown by scanning and transmission electron microscopy (SEM, TEM) to result in collapsed surface collagen (Sugizaki 1991; Inokoshi et al., 1993; Gwinnett, 1994; Dickens, 1995). Pashley (1993) reported that conditioning hard tissues with aqueous phosphoric acid (H.sub.3 PO.sub.4) and a subsequent drying step caused reduced porosity of the upper demineralized dentin and produced a dense collagen crust. He also observed that collagen collapsed to a certain degree even on surfaces that had been kept moist after conditioning and hypothesized that the surrounding water, although hydrogen-bonded to the collagen, was not strong enough to support the mineral-depleted collagen in the same way as original dentin mineral.
Various approaches to preventing collapse of surface collagen have been published: Sugizaki (1991) showed that treatment of conditioned, dried dentin with various hydrophllic monomers re-expanded the collapsed collagen to its original level. Conditioning with 10%.sup.1 citric acid containing either 20% calcium chloride or 3% ferric chloride (termed `10-3`), was effective in preventing the collapse of surface collagen (Sugizaki, 1991). Nakabayashi (1985) speculated that ferric chloride suppresses the denaturation of collagen fibers, thereby contributing to higher bond strength. The latter point was supported by Mizunuma (1986), who reported that collagen fibers treated this way are less susceptible to trypsin digestion. The theory was questioned by Imai et al. (1991), who suggested that ferric ions adsorbed onto collagen may act as polymerization initiators and accelerators. TE micrographs of dentin, which had been conditioned with 10-3 and then dried, showed deposition of electron-dense material along extended collagen fibrils (Dickens, 1995) and confirmed Sugizaki's observations. Precipitated Fe- and/or Ca-salts had strengthened the collagen fibrils sufficiently to prevent them from collapsing when dried. When similarly treated specimens were tested in a shear bond test, they still showed significantly lower SBS than specimens for which dentin surfaces were kept moist after conditioning. That suggested that other parameters, e.g., decreased wetting of the dried surface, may have resulted in less complete infiltration of the primer.
Infiltration ofthe primer in the acid-treated dentin surface to a depth less than that altered by the acidic conditioner is thought to be responsible for a potentially weak collagen-rich band between conditioned and unaltered dentin. (Kiyomura, 1987; Nakabayashi 1995; Dickens-Venz et al., 1992; Van Meerbeck et al., 1992; Tam and Pilliar, 1994). Use of acidic polymerizable primers e.g. phenyl-P (2-methacryloyl phenyl hydrogen phosphate) in 2-hydroxyethyl methacrylate (HEMA; Watanabe et al., 1994) or 2-acryloyloxyethyl hydrogen maleate in water (Inoue et al., 1993) without any additional conditioners, was reported to have penetrated throughout the smear layer and formed with the unaltered dentin an acid-proof, hybridized layer.
Several currently used bonding systems use primers based on carboxylic acid monomers (Bowen, 1965; Bowen et al., 1982; Bowen, 1985; Bowen et al.; 1987; Bowen, 1994; Suh et al., 1994), e.g., PMDM (reaction product of pyromellitic dianhydride (PMDA) and HEMA) and are combined with a second primer, an N-compound that has surface-active properties, e.g., N-phenylglycine or magnesium bis-(N-p-tolylglycine glycidylmethacrylate) [Mg(NTG-GMA).sub.2 ]. Bonding to dentin with these systems is achieved by conditioning the dentin surface and then coating it with a mixture of the two primers, also called adhesion promoters. The priming resin that is currently used in these bonding systems is PMGDM, which is the addition reaction product of PMDA and glycerol dimethacrylate (Venz and Dickens, 1993; Bowen, 1994). The primer is activated by combining in a dappen dish 40 .mu.L (two drops) of a 20% solution of PMGDM in acetone and 20 .mu.L (one drop) of an acetone solution of 5% Mg(NTG-GMA).sub.2. This mixture is brushed onto the conditioned surface. An unfilled bonding resin that may consist of bis-GMA (2,2-bis[p(2'-hydroxy-3'-methacryloxypropoxy)phenylene]propane) and HEMA is placed on the primed surface, thinned with a stream of air and light-cured. A composite resin is adapted to the prepared surface and also light-cured.
In some investigations acidic, carboxylated monomers in combination with other hydrophilic monomers for priming tooth surfaces have been used. Fukushima et al. (1985) reported syntheses of such monomers by reacting 2,2-bis[p(2'-hydroxy-3'-methacryloxypropoxy)phenylene]propane (bis-GMA) and other hydroxylated monomers with the aliphatic succinoxy anhydride resulting in a compound with two aliphatic carboxylic acid groups. Their approach was to use these compounds as primers on dentin and enamel with or without prior conditioning of the tooth surfaces with 37% phosphoric acid. The mean tensile bond strengths between 2 and 12 MPa were considerably lower than shear bond strengths obtainable with the invention presented here.
The 3M company (Minnesota Manufacturing and Mining, St. Paul, Minn.) has developed bonding systems (trade names: Scotchbond.RTM., SB2; SBMP; U.S. Pat. Nos. 4,553,941 1985; 4,719,149 1988; 4,880,660 1989; 5,554,030 and 5525,648 1996) with primers that contain HEMA and maleic acid. Although maleic acid has a polymerizable functionality, it cannot polymerize without the more reactive HEMA. Therefore, if the maleic acid penetrates into the dentin more deeply than the hydrophilic monomer, the above described phenomenon of nonimpregnated collagen may occur. This incidence has been observed by transmission electron microscopy when Scotchbond 2 was applied to dentin (Dickens-Venz et al., 1992). Since both maleic acid and HEMA are monofunctional monomers, upon polymerization they form a linear polymer.
Organic phosphonates used to promote adhesion to enamel were described by Anbar and Farley (1974), who added vinyl phosphonic acid (VPA) or vinylbenzyl phosphonic acid to composite resins. The authors also claimed that precoating of enamel with a 0.12% neutrazed solution of the said acids improved the bond strength significantly. However, in contrast to the approach taken in this invention, they ensured that the precoating had no etching effect on the enamel.
Acrylated phosphonate esters described for use as adhesion promoting agents for dentin and hard tissues were used as primers and/or admixed with composite resins (Cabasso and Sahni, 1990). The shear bond strengths obtained with such modified composites ranged from 2 MPa to 7 MPa.
A series of primers based on dipentaerythritol-pentaacrylate phosphate esters was developed by Dentsply Int., (trade names: Prisma Universal Bond, PUB, PUB2, PUB 3; U.S. Pat. Nos. 4,514,342, 1985; 4,6657,941 1987; 4,814,423 1989; 4,966,934 1990). These primers are applied to unconditioned dentin. Since they are only weakly acidic compounds, they do not completely remove the smear layer and cause only minor, if any, subsurface demineralization and subsequent formation of hybridized dentin. Mean shear bond strengths of Prisma Universal Bond 2.RTM. were reported to be about 7 MPa (Stangel et al., 1994).
The dentin and enamel priming agent of the bonding system Optibond.RTM. (Kerr, Santa Ana, Calif.) contains HEMA, a HEMA-phthalate derivative with carboxylic-acid functionality, and glycerophosphate dimethacrylate. This agent is a primer and is applied to acid-etched dentin or enamel (Van Meerbeck et al., 1996). That is, the Optibond.RTM. primer acts on already demineralized tooth substrates. In addition, like other priming agents, it remain on the tooth surface, leaving dissolved and loosely bound matter.
There is a need in the art for new methods and compositions for restoring teeth in which the bond strength is good and the time required for the restoration is short.